As an optical subscriber transmission system which economically provides a narrow-band service such as a telephone, a passive optical network(PON) system has been proposed and developed. The PON system has, as shown in FIG. 1, a passive double star(PDS) composition in which a one-to-N optical transmission system between a central office 1 and optical network units(ONU)11 to 1N of number N performs distribution/multiplexing through a passive splitter 3. The user multiplexing is performed such that a downstream signal is treated by time division multiplexing(TDM) and a upstream signal is treated by time division multiple access(TDMA). Thus, the optical transmission line between the central office 1 and the splitter 3 and an optical transmitter-receiver 2 in the central office 1 are shared by ONUs 11 to 1N of number N to economize.
Since the PON system is directed to the narrow-band service such as a telephone, to apply it to a broad-band service such as video or movie, it should have an increased transmission capacity while utilizing the existing optical transmission line and optical transmitter-receiver as much as possible.
In the conventional method for increasing the transmission capacity while utilizing the existing equipment, using wavelength division multiplexing(WDM) and enhancing the degree of TDM are proposed. One way of providing WDM and enhanced TDM is, as shown in FIG. 2, that a wavelength not used for the narrow-band service is assigned to each ONU or each service by using the high-density wavelength multiplexing technique another way is that a high clock rate burst for the broad-band service is transmitted mixed with a low clock rate burst. For example, this is reported in the 5th Conference on Optical/Hybrid Access Networks, lecture No. 6.03 and the 6th International Workshop on Optical Access Networks, lecture No. 2.4. In FIG. 2, the PON system comprises the central office 1 and ONUs 11 to 1N connected thereto by the optical transmission lines 51 to 5N, wherein the central office 1 comprises the single core type optical transmitter-receivers 71 and 72, the passive splitter 3, the wavelength multiplexing and demultiplexing circuit 7 connecting each of the optical transmitter-receivers 71 and 72 to the passive splitter 3, and the optical transmitter-receiver 2, and ONU 11 comprises the optical transmitter-receiver 31 and the optical transmitter-receiver 41 having a function of wavelength multiplexing.
On the other hand, in the PON system for the narrow-band service, an example that the splitter is disposed in the central office has also been proposed. In this case, without the relaying of a transmission line, ONUs of number N and optical transmitter-receivers of number N in the central office are, as shown in FIG. 3, connected one-to-one, respectively, to form a single star topology. Therefore, the transmission capacity of each ONU can be increased. In FIG. 3, the PON system comprises the central office 1 and ONUs 11 to 1N connected thereto by the optical transmission lines 51 to 5N, respectively, wherein the central office 1 comprises optical transmitter-receivers 21 to 2N, and ONU 11 to 1N comprise the optical transmitter-receivers 31 to 3N, respectively.
However, in the conventional optical fiber network system, to add WDM to the existing PON system to provide the broad-band service, an expensive light source for WDM and an expensive wavelength selecting device as well as the temperature control therefor, which needs a large consumed power, are required. Namely, as compared with the case of originally laying the single star topology, the conventional optical fiber network system with WDM needs more expensive and larger central office equipment and ONUs.
Furthermore, in the case of using the high-speed burst mixed with the low-speed burst, a burst mode compatible receiver which has a reception sensitivity worse than that of a receiver for a continuous signal needs to be sped up. Therefore, it is very difficult for the receiver to get a practical reception sensitivity.
In the case of shifting to the single star topology, it is impossible for the total number of optical transmitter-receivers of numbers required in PON to be accommodated in the central office device designed for PON when utilizing the single star topology.